This invention relates generally to radio frequency amplifier circuits and more particularly to such circuits having means for compensating for a variations in gain over a given frequency range, such as may be caused by variations in temperature.
As is known in the art, FET amplifiers, often used in distributed amplifier circuits, commonly exhibit variations in gain as a function of frequency, as may be caused by temperature variations. More specifically, two types of gain variations occur. The first may be referred to as gain offset and is a phenomena in which the overall gain magnitude curve is offset upward or downward as a result of temperature variations. This type of gain variation is generally tolerable since it is readily compensated. The second type of gain variations with temperature are those which render the gain characteristic sloped within a given frequency band. This latter type of variation may be undesirable in cases where system requirements call for a "flat" input to output gain response (i.e. a gain response having zero slope) within the given frequency band. For example, requirements for ECM transmitters, receivers, and repeaters often require a flat gain response even with variations in temperature.
One way known in the art to compensate for gain variations caused by temperature changes, is to use an attenuator controlled by a thermistor network. A thermistor is a device having a resistance which varies in accordance with temperature. By controlling the attenuator with a thermistor network, the attenuation of the gain varies in accordance with temperature variations. However, this type of compensation adjusts, or more specifically, offsets the overall gain magnitude curve upward or downward in accordance with temperature and thus addresses the former type of gain variations. That is, the gain, along with its undesirable variations within a given frequency range, moves upward or downward. Thus, the resulting gain versus frequency curve remains sloped within the given frequency band as a result of temperature variations.